U.S. patent application number 14/916162 was filed with the patent office on 2016-07-07 for photoacoustic apparatus.
The applicant listed for this patent is CANON KABUSHIKI KAISHA. Invention is credited to Robert A Kruger.
Application Number | 20160192843 14/916162 |
Document ID | / |
Family ID | 52472573 |
Filed Date | 2016-07-07 |
United States Patent
Application |
20160192843 |
Kind Code |
A1 |
Kruger; Robert A |
July 7, 2016 |
PHOTOACOUSTIC APPARATUS
Abstract
A photoacoustic apparatus disclosed in the description includes
a light source; transducers that detect acoustic waves and output
electric signals, the acoustic waves being generated when an object
is irradiated with light generated from the light source; a support
member that supports the transducers such that directional axes of
the transducers gather; a moving unit that moves the support member
relative to the object within a movement region; a storage unit
that stores the electric signals output from the transducers at
timings; and a computing unit that acquires object information for
each reconstruction position on the basis of the electric signals
stored in the storage unit. The light source generates the light at
the timings. The moving unit moves the support member such that
there exists a region in which a density of a distribution of
positions of the support member at the timings is constant.
Inventors: |
Kruger; Robert A; (Oriental,
NC) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
CANON KABUSHIKI KAISHA |
Tokyo |
|
JP |
|
|
Family ID: |
52472573 |
Appl. No.: |
14/916162 |
Filed: |
September 3, 2014 |
PCT Filed: |
September 3, 2014 |
PCT NO: |
PCT/US14/53820 |
371 Date: |
March 2, 2016 |
Related U.S. Patent Documents
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
|
|
61873542 |
Sep 4, 2013 |
|
|
|
Current U.S.
Class: |
600/407 |
Current CPC
Class: |
A61B 5/14542 20130101;
A61B 5/0095 20130101; G01N 21/1702 20130101; A61B 5/4312 20130101;
G01N 29/2418 20130101 |
International
Class: |
A61B 5/00 20060101
A61B005/00 |
Claims
1. A photoacoustic apparatus comprising: a light source; a
plurality of transducers configured to detect acoustic waves and
output electric signals, the acoustic waves being generated when an
object is irradiated with light generated from the light source; a
support member configured to support the plurality of transducers
such that directional axes of the plurality of transducers gather;
a moving unit configured to move the support member relative to the
object within a movement region; a storage unit configured to store
the electric signals output from the plurality of transducers at a
plurality of timings; and a computing unit configured to acquire
object information for each reconstruction position on the basis of
the electric signals stored in the storage unit, wherein the light
source generates the light at the plurality of timings, and wherein
the moving unit moves the support member such that there exists a
region in which a density of a distribution of positions of the
support member at the plurality of timings is constant.
2. The photoacoustic apparatus according to claim 1, wherein the
moving unit moves the support member such that distances between
adjacent positions of the support member among the positions of the
support member at the plurality of timings are equal to each
other.
3. The photoacoustic apparatus according to claim 2, wherein the
moving unit moves the support member such that the distances
between the adjacent positions of the support member are within
.+-.20% of an average distance between the adjacent positions of
the support member in the movement region.
4. The photoacoustic apparatus according to claim 1, wherein the
moving unit moves the support member such that distances from each
position of the support member at the plurality of timings to at
least three positions of the support member adjacent thereto are
equal to each other.
5. The photoacoustic apparatus according to claim 4, wherein the
moving unit moves the support member such that the distances from
each position of the support member at the plurality of timings to
the at least three positions of the support member adjacent thereto
are within .+-.10% of an average of the distances.
6. The photoacoustic apparatus according to claim 1, wherein the
moving unit moves the support member such that a distribution of
positions of the support member at a light irradiation timing in
each of four regions defined by two orthogonal planes passing
through a center of the movement region is uniform.
7. The photoacoustic apparatus according to claim 6, wherein the
moving unit moves the support member such that an average
difference in the number of positions of the support member at the
light irradiation timing in each of the four regions is within
.+-.20%, and an average difference in distance between adjacent
positions of the support member is within .+-.20%.
8. The photoacoustic apparatus according to claim 1, wherein the
moving unit moves the support member such that in each of four
regions defined by two orthogonal planes passing through a center
of the movement region, the number of positions of the support
member at a light irradiation timing is plural.
9. The photoacoustic apparatus according to claim 1, wherein the
computing unit acquires the object information for each
reconstruction position using, among the electric signals stored in
the storage unit, electric signals output from the plurality of
transducers at a part of the plurality of timings.
10. The photoacoustic apparatus according to claim 1, wherein the
computing unit acquires the object information for each
reconstruction position using, among the electric signals stored in
the storage unit, electric signals corresponding to acoustic waves
having a same wave-number vector.
11. The photoacoustic apparatus according to claim 1, wherein the
moving unit continuously moves the support member.
12. The photoacoustic apparatus according to claim 11, wherein the
light that is generated by the light source has a constant period,
and wherein the moving unit moves the support member at a constant
speed in a tangential direction of a movement path of the support
member.
13. The photoacoustic apparatus according to claim 1, wherein the
moving unit continuously moves the support member in a circular
movement path.
14. The photoacoustic apparatus according to claim 1, wherein the
moving unit moves the support member such that coordinates in a
radial direction with respect to a center of the movement region
either increase or decrease.
15. The photoacoustic apparatus according to claim 1, wherein the
moving unit moves the support member in a spiral-pattern movement
path in which a radius increases or decreases with time.
16. The photoacoustic apparatus according to claim 15, wherein the
moving unit moves the support member from an outer periphery
towards an inner periphery of the spiral-pattern movement path.
17. The photoacoustic apparatus according to claim 1, wherein the
moving unit moves the support member in a plurality of concentric
movement paths having different radii.
18. The photoacoustic apparatus according to claim 1, wherein the
support member is hemispherical in shape.
19. The photoacoustic apparatus according to claim 1, further
comprising an optical system configured to guide the light
generated from the light source to a position where the directional
axes gather, wherein the moving unit moves the support member and
the optical system in synchronism with each other.
20. The photoacoustic apparatus according to claim 1, wherein the
support member supports the plurality of transducers such that the
plurality of transducers are arranged in a three dimensional
space.
21. The photoacoustic apparatus according to claim 1, wherein the
moving unit moves the support member so that a density of
distribution of positions of the support member at a light
irradiation timing corresponding to a set region of interest is
constant.
22. A photoacoustic apparatus comprising: a light source; a
plurality of transducers configured to detect acoustic waves and
output electric signals, the acoustic waves being generated when an
object is irradiated with light generated from the light source; a
support member configured to support the plurality of transducers
such that directional axes of the plurality of transducers gather;
a moving unit configured to move the support member relative to the
object within a movement region; a storage unit configured to store
the electric signals output from the plurality of transducers at a
plurality of timings; and a computing unit configured to acquire
object information for each reconstruction position on the basis of
the electric signals stored in the storage unit, wherein the
support member has a space disposed between the support member and
the object and capable of being filled with an acoustic matching
material, and wherein the moving unit continuously moves the
support member in a circular movement path.
23. The acoustic apparatus according to claim 22, wherein the
moving unit moves the support member in a spiral-pattern movement
path in which a radius increases or decreases with time.
24. The acoustic apparatus according to claim 23, wherein the
moving unit moves the support member from an outer periphery
towards an inner periphery of the spiral-pattern movement path.
25. The acoustic apparatus according to claim 22, wherein the light
that is generated by the light source has a constant period, and
wherein the moving unit moves the support member at a constant
speed in a tangential direction of a movement path of the support
member.
Description
TECHNICAL FIELD
[0001] The present invention relates to a photoacoustic apparatus
that acquires information regarding the interior of an object by
making use of a photoacoustic effect.
BACKGROUND ART
[0002] Studies of optical imaging apparatuses have been actively
conducted in the field of medicine. The optical imaging apparatuses
irradiate an object (such as a living body) with light from a light
source (such as a laser) and form an image from information
regarding the interior of the object, the information being
acquired on the basis of incident light. Photoacoustic imaging
(PAI) is one of such optical imaging techniques. In the
photoacoustic imaging, an object is irradiated with pulsed light
generated from a light source, acoustic waves (typically ultrasonic
waves) generated from tissues of the object that absorb energy of
the pulsed light that has propagated and that has been diffused in
the object are detected, and information regarding the interior of
the object is subjected to imaging on the basis of the detection
signals. That is, by making use of a difference in the rate of
absorption of optical energy between a target area (such as a
tumor) and other tissues, an acoustic wave detector receives
elastic waves (photoacoustic waves) generated when a test area
momentarily expands by absorbing optical energy with which the test
area is irradiated. By mathematically processing the detection
signals, it is possible to acquire information regarding the
interior of the object. In recent years, the photoacoustic imaging
has been used to actively conduct preclinical studies in which
blood vessels of small animals are imaged, and clinical studies in
which the principle of the photoacoustic imaging is applied to the
diagnosis of, for example, breast cancer ("Photoacoustic imaging in
biomedicine", M. Xu, L. V. Wang, REVIEW OF SCIENTIFIC INSTRUMENT,
77, 041101, 2006).
[0003] However, it is desired that photoacoustic apparatuses more
efficiently and precisely acquire object information.
CITATION LIST
Non Patent Literature
[0004] NPL 1 Photoacoustic Imaging in Biomedicine
SUMMARY OF INVENTION
[0005] The present invention provides a photoacoustic apparatus
that is capable of efficiently and precisely acquiring object
information.
[0006] A photoacoustic apparatus that is disclosed in the
description includes a light source; a plurality of transducers
configured to detect acoustic waves and output electric signals,
the acoustic waves being generated when an object is irradiated
with light generated from the light source; a support member
configured to support the plurality of transducers such that
directional axes of the plurality of transducers gather; a moving
unit configured to move the support member relative to the object
within a movement region; a storage unit configured to store the
electric signals output from the plurality of transducers at a
plurality of timings; and a computing unit configured to acquire
object information for each reconstruction position on the basis of
the electric signals stored in the storage unit. The light source
generates the light at the plurality of timings. The moving unit
moves the support member such that there exists a region in which a
density of a distribution of positions of the support member at the
plurality of timings is constant.
[0007] Further features of the present invention will become
apparent from the following description of exemplary embodiments
with reference to the attached drawings.
BRIEF DESCRIPTION OF DRAWINGS
[0008] FIG. 1 schematically illustrates an exemplary configuration
of a photoacoustic apparatus according to an embodiment.
[0009] FIG. 2 schematically illustrates an exemplary movement of a
support member in the embodiment.
[0010] FIG. 3 illustrates an exemplary movement path of the support
member in the embodiment.
[0011] FIG. 4 illustrates another exemplary movement path of the
support member in the embodiment.
[0012] FIG. 5 illustrates a distribution of measurement positions
in the embodiment.
[0013] FIG. 6 illustrates a photoacoustic apparatus according to
Example.
[0014] FIG. 7 schematically illustrates part of a configuration of
the photoacoustic apparatus according to Example.
[0015] FIG. 8 schematically illustrates an exemplary configuration
of the photoacoustic apparatus according to Example.
[0016] FIG. 9 illustrates distributions of measurement positions in
the photoacoustic apparatus according to Example.
[0017] FIG. 10 illustrates photoacoustic images obtained by the
photoacoustic apparatus according to Example.
[0018] FIG. 11 illustrates a phantom used in Example.
[0019] FIG. 12 illustrates photoacoustic images obtained by the
photoacoustic apparatus according to Example.
[0020] FIGS. 13A and 13B show a result of analysis of the
photoacoustic images obtained by the photoacoustic apparatus
according to Example.
[0021] FIG. 14 illustrates a different photoacoustic image obtained
by the photoacoustic apparatus according to Example.
[0022] FIG. 15 illustrates a different photoacoustic image obtained
by the photoacoustic apparatus according to Example.
DESCRIPTION OF EMBODIMENTS
[0023] A configuration of a photoacoustic apparatus 1 according to
an embodiment is described with reference to FIG. 1.
[0024] The photoacoustic apparatus 1 according to the embodiment
includes a light source 11, an optical system 13, a plurality of
transducers 17 that are supported by a support member 22, a
computer 19, a display device 20, and a scanner 21.
[0025] Pulsed light 12 emitted from the light source 11 is guided
while being processed into a desired light distribution shape by
the optical system 13 including, for example, a lens, a mirror, an
optical fiber, and a diffusing plate. Then, the pulsed light 12 is
applied to an object 15, such as a living body. At a timing in
which the pulsed light 12 is applied, the pulsed light 12 reaches
the entire interior of the object 15 at substantially the same
time. When energy of the pulsed light 12 that has propagated
through the interior of the object 15 is partly absorbed by a light
absorber 14 (which eventually becomes a sound source), such as a
blood vessel containing a large amount of hemoglobin, photoacoustic
waves (typically ultrasonic waves) 16 are generated by thermal
expansion of the light absorber 14. The photoacoustic waves 16
propagate through the interior of the object 15 and an acoustic
matching material 18, and reach the plurality of transducers 17
supported by the support member 22. The plurality of transducers 17
receive the photoacoustic waves 16 and convert them into electric
signals.
[0026] While the scanner 21 moves the support member 22, the
photoacoustic waves are measured at a plurality of timings. The
term "measure" in the description refers to application of light
and reception of photoacoustic waves generated by the application
of light. The term "measurement position" refers to the position of
a search unit when light is applied, that is, the position of the
support member 22. In the embodiment, a center position of the
support member 22 at a light irradiation timing serves as a
measurement position.
[0027] Typically, the speed of propagation of the photoacoustic
waves 16 is faster than the speed at which the scanner 21 moves the
support member 22. Therefore, the photoacoustic waves 16 are
received at positions of the plurality of transducers 17 at the
timing in which the pulsed light 12 is applied. Here, the movement
of the support member 22 from the time when the pulsed light 12 is
applied to the object 15 to the time when the plurality of
transducers 17 detect the photoacoustic waves 16 can be ignored.
Therefore, in the present embodiment, the timing in which the
pulsed light 12 is applied corresponds to the timing in which the
photoacoustic waves 16 are measured (hereunder referred to as
"measurement timing"). In addition, the positions taken by the
plurality of transducers 17 at the timing in which the pulsed light
12 is applied correspond to photoacoustic-wave measurement
positions that can be taken at the timing in which the
photoacoustic waves 16 are measured. Since the support member 22
supports the plurality of transducers 17, the positions of the
plurality of transducers 17 can be specified by specifying the
position of the support member 22.
[0028] Next, electric signals output from the plurality of
transducers 17 at each timing are amplified and converted into
digital signals by the computer 19, and are stored in a storage
unit in the computer 19. That is, the storage unit in the computer
19 stores the electric signals output from the plurality of
transducers 17 at different timings.
[0029] Then, the computer 19 acquires object information for each
reconstruction position in a region subjected to imaging using the
electric signals output from the plurality of transducers at the
different timings. A reconstruction position where the object
information is acquired is a voxel in the case where
three-dimensional information is acquired, and is a pixel in the
case where two-dimensional information is acquired. Known
reconstruction techniques, such as universal back projection (UBP)
and filtered back projection (FBP), can be used to acquire object
information from the electric signals. According to these
reconstruction techniques, it is possible to acquire object
information for one position from the electric signals obtained at
the different timings.
[0030] In the present embodiment, the entire object is set as a
region to be subjected to imaging. A region that has been
previously set or that has been set by a user using an input unit
can be set as a region to be subjected to imaging.
[0031] Next, the computer 19 generates image data for being
displayed on the display device 20 from the acquired object
information.
[0032] Next, the computer 19 displays the image data on the display
device 20. The image of the object information displayed on the
display device 20 in this way can be used for, for example,
diagnostic purposes.
[0033] To acquire object information for each position in a region
to be subjected to imaging, the computer 19 need not use all
electric signals stored in the storage unit. That is, from among
the electric signals stored in the storage unit, the computer 19
may use electric signals output from the plurality of transducers
17 at a part of the timings to acquire object information for each
position in the region to be subjected to imaging. For example,
electric signals used to acquire object information may be
determined on the basis of the directionality of the transducers or
the light value for each position at each timing.
[0034] In the present embodiment, since wave-number information can
be efficiently acquired, it is possible to precisely acquire object
information from a large amount of wave-number information for each
position even in the case where electric signals obtained at a part
of the timings are used. In addition, since the number of electric
signals used to acquire object information is reduced, it is
possible to reduce the time required to acquire the object
information.
[0035] Examples of the object information that can be acquired by
the photoacoustic apparatus according to the embodiment include a
distribution of initial sound pressures of photoacoustic waves, a
distribution of optical energy absorption densities, a distribution
of absorption coefficients, and a distribution of concentrations of
materials that form the object. The concentrations of materials
include a degree of oxygen saturation, an oxyhemoglobin
concentration, a deoxyhemoglobin concentration, and a total
hemoglobin concentration. The total hemoglobin concentration is the
sum of the concentrations of oxyhemoglobin and deoxyhemoglobin.
Each Component of Photoacoustic Apparatus
[0036] Next, each component of the photoacoustic apparatus
according to the present embodiment is described in detail.
Light Source 11
[0037] The light source 11 supplies optical energy to the object 15
and causes the photoacoustic waves 16 to be generated. When the
object 15 is a living body, the light source 11 emits light of a
specific wavelength to be absorbed by a specific one of components
of the object 15. It is desirable that the light source 11 be a
pulsed light source that can generate pulsed light of the order of
from a few to a few hundred nanoseconds as irradiation light. More
specifically, it is desirable to use a pulse width of approximately
10 to 100 nanoseconds to efficiently generate photoacoustic waves.
It is desirable to use laser as the light source 11 to achieve high
output. However, a light-emitting diode or the like may be used
instead of the laser. Various lasers, such as a solid-state laser,
a gas laser, a fiber laser, a dye laser, and a semiconductor laser,
may be used for the laser. For example, the irradiation timing,
waveform, and intensity are controlled by a light-source controller
(not shown). When the object 15 is a living body, it is desirable
that the wavelength of the light source 11 used be one that allows
light to propagate to the interior of the living body.
Specifically, the wavelength may range from 500 nm to 1200 nm.
[0038] The light source 11 may be provided separately from the
photoacoustic apparatus. In addition, the light source 11 may be
formed by either a single light source or a plurality of light
sources.
Optical System 13
[0039] The pulsed light 12 emitted from the light source 11 is
guided to the object 15 while being processed into a desired light
distribution shape typically by the optical system 13 including,
for example, a lens or a mirror. For example, an optical waveguide
such as an articulating arm formed by mounting a mirror or the like
in a lens barrel, optical fibers, and a bundle of optical fibers
are regarded as components of the optical system 13. Examples of
other components of the optical system 13 include a mirror that
reflects light, a lens that converges or diverges light or changes
the shape of light, and a diffusing plate that diffuses light. Any
optical components may be used, as long as the pulsed light 12
emitted from the light source 11 is applied to the object 15 in a
desired shape. It is desirable to diverge the pulsed light 12 by
the lens over a larger area, rather than converging the pulsed
light 12 by the lens, from the viewpoint of expanding a region of
the object 15 to be diagnosed.
[0040] The photoacoustic apparatus need not include the optical
system 13 if a pulsed light 12 that is desired pulsed light is
emitted from the light source 11.
Object 15 and Light Absorber 14
[0041] The object 15 and the light absorber 14 are now described,
although they do not constitute part of the photoacoustic apparatus
according to the present embodiment. The photoacoustic apparatus
according to the present embodiment is primarily used, for example,
for diagnosis of malignant tumors or blood vessel diseases of
humans or animals, and follow-up of chemotherapy. Therefore, the
object 15 may be an area of a human or animal body to be diagnosed,
such as a breast, a finger, an arm, or a leg. The light absorber 14
inside the object 15 has a relatively high absorption coefficient
therein. For example, when a human body is an object to be
measured, the light absorber 14 may be oxygenated or reduced
hemoglobin, or a blood vessel or a newborn blood vessel containing
a large amount of oxygenated or reduced hemoglobin. The light
absorber 14 on the surface of the object 15 may be, for example,
melanin. By selecting an appropriate wavelength of light, other
materials, such as fat, water, and collagen, may serve as the light
absorber 14 in a human body.
Transducer 17
[0042] A transducer 17 receives acoustic waves and converts them
into electric signals which are analog signals. The transducer 17
may be any transducer as long as it detects photoacoustic waves,
such as a transducer that uses a piezoelectric effect, a transducer
that uses resonance of light, and a transducer that makes use of
changes in capacitance. A plurality of transducers 17 are arranged
in the present embodiment. By the use of such multi-dimensionally
arranged elements, acoustic waves can be received simultaneously at
multiple locations. This can reduce measurement time and reduce the
influence of, for example, vibration of the object 15.
Support Member 22
[0043] The support member 22 supports the plurality of transducers
17 along the support member 22. FIG. 1 is a sectional view of the
support member 22 in an x-z plane thereof.
[0044] It is desirable that the support member 22 support the
plurality of transducers 17 along a closed surface that surrounds
the object 15. However, when the object 15 is, for example, a human
body and it is difficult to arrange the plurality of transducers 17
on all closed surfaces that surround the object 15, it is desirable
to arrange the plurality of transducers 17 on the surface
(hemispherical surface) of the hemispherical support member 22
having an opening as in the present embodiment.
[0045] It is desirable that the plurality of transducers 17 on the
support member 22 be arranged such that sampling can be performed
at equal intervals in a k-space. For example, it is desirable that
the plurality of transducers 17 be arranged in a spiral pattern as
described in U.S. Pat. No. 5,713,356.
[0046] In general, a normal direction to a receiving surface (front
surface) of a transducer is a direction of highest receiving
sensitivity. By causing axes (hereunder referred to as "directional
axes") along the directions of highest receiving sensitivity of the
plurality of transducers 17 to gather towards the center of
curvature of the hemisphere, a region that can be formed into a
visible region is formed with high precision near the center of
curvature. Particularly, in the present embodiment, the plurality
of transducers 17 are arranged such that the directional axes of
the respective transducers intersect the center of curvature of the
hemisphere. This can increase the resolution of a region where the
directional axes gather. In the description, such a region of high
resolution is referred to as a high-resolution region 23. In the
present embodiment, the high-resolution region 23 refers to a
region that extends from the point of highest resolution to the
point at which the resolution is half the highest resolution. Note
that as long as the directional axes gather to a specific region
and the desired high-resolution region 23 can be formed, the
directional axes of the respective transducers need not intersect
with each other.
[0047] FIG. 1 illustrates an exemplary arrangement of the
transducers, and the way of arrangement of the transducers is not
limited thereto. The transducers may be arranged in any way as long
as the directional axes can be gathered to a specific region and a
desired high-resolution region can be formed. That is, the
plurality of transducers 17 may be arranged along a curved shape so
as to form a desired high-resolution region. Further, in the
description, the term "curved surface" also refers to a spherical
surface or a spherical surface having an opening, such as a
hemispherical surface. In addition, the term "curved surface" also
refers to an uneven surface that is uneven to the extent that
allows it to be considered as a spherical surface and a surface of
an ellipsoid (which is a three-dimensional analog of an ellipse and
has a two-dimensional curved surface) that is elliptical to the
extent that allows it to be considered as a spherical surface.
[0048] When the plurality of transducers 17 are arranged along the
support member 22 having a shape formed by arbitrarily sectioning a
sphere, the directional axes maximally gather at the center of
curvature of the support member. The hemispherical support member
22 that is described in the embodiment is also an example of a
support member having a shape formed by arbitrarily sectioning a
sphere. In the description, a shape that is formed by arbitrarily
sectioning a sphere refers to a shape based on a sphere.
Accordingly, the plurality of transducers that are supported by the
support member having such a shape based on a sphere are supported
on a sphere.
[0049] For example, other curved or piecewise linear surfaces can
also be used as the support member 22.
[0050] It is desirable that the support member 22 have a space that
can be filled with the acoustic matching material 18.
Acoustic Matching Material 18
[0051] The acoustic matching material 18 is an impedance matching
material that can fill a space between the object 15 and the
plurality of transducers 17 and that acoustically couples the
object 15 and the plurality of transducers 17. As the acoustic
matching material 18, it is desirable to use a material whose
acoustic impedance is close to those of the object 15 and the
transducers 17 and that transmits pulsed light therethough. In
addition, it is desirable that the acoustic matching material 18 be
a liquid or a gas that does not prevent the movement of the support
member 22. More specifically, the acoustic matching material 18 may
be, for example, water, castor oil, or gel.
Computer 19
[0052] The computer 19 is capable of performing predetermined
processing on electric signals output from the plurality of
transducers 17. The computer 19 is capable of controlling the
operation of each component of the photoacoustic apparatus. The
computer 19 is capable of setting a desired measurement position.
That is, the computer 19 is capable of providing a desired
measurement position by controlling the timing in which the light
source 11 emits light and driving of the scanner 21 that moves the
support member 22.
[0053] A computing unit in the computer 19 typically includes an
element, such as a central processing unit (CPU), a graphics
processing unit (GPU), or an analog-to-digital (A/D) converter; or
a circuit, such as a field programmable gate array (FPGA) or an
application specific integrated circuit (ASIC). The computing unit
may be formed not only by a single element or circuit, or but also
by a plurality of elements or circuits. Also, each processing
operation performed by the computer 19 may be performed by any of
the elements or circuits.
[0054] The storage unit in the computer 19 typically includes a
storage medium, such as a read-only memory (ROM), a random-access
memory (RAM), or a hard disk. The storage unit may be formed not
only by a single storage medium, but also by a plurality of storage
media.
[0055] It is desirable that the computer 19 be configured to
perform pipeline processing of a plurality of signals at the same
time. This can reduce the time necessary to acquire object
information.
[0056] Each processing operation performed by the computer 19 can
be stored in the storage unit as a program to be executed by the
computing unit. Note that the storage unit where the program is
stored is a non-transitory recording medium.
Display Device 20
[0057] The display device 20 is a device that displays image data
output from the computer 19. Although a liquid crystal display or
the like is typically used as the display device 20, a plasma
display, an organic electro-luminescent (EL) display, or a field
emission display (FED) may also be used. The display device 20 may
be provided separately from the photoacoustic apparatus.
Scanner 21
[0058] The scanner 21, serving as a moving unit, moves the support
member 22 relative to the object 15. Since the plurality of
transducers 17 arranged on the support member 22 can be moved
relative to the object 15 by the scanner 21, the photoacoustic
waves 16 can be received at a plurality of measurement
positions.
[0059] It is desirable that the scanner 21 cause the support member
22 to undergo circular movement. The term "circular movement"
refers to a curvilinear movement similar to a circular movement and
an elliptical movement. It is desirable that the scanner 21 move
the support member 22 such that coordinates in a radial direction
with respect to the center of a movement region either increase or
decrease.
[0060] FIG. 2 schematically illustrates an exemplary circular
movement. Referring to FIG. 2, a point o is a movement plane center
24, a circle represents a movement path of a position of the
support member 22, and a point p is a point on the movement path of
the position of the support member 22. This movement gives a speed
in a radial direction (radial speed) v.sub.r and a speed in a
tangential direction (tangential speed) v.sub.t to the position of
the support member 22 at the point p. Position coordinates (x, y)
of the point p in a polar coordinate system can be expressed by
Equation (1) below:
[ Math . 1 ] { x = r cos .phi. y = r sin .phi. Equation ( 1 )
##EQU00001##
where r is a coordinate in the radial direction (movement radius),
and .phi. is an angle formed between the x axis and a line
extending from the origin to the point p. In the present
embodiment, the scanner 21 moves the support member 22 such that
coordinates (r) on the movement path of the position of the support
member 22 in the radial direction either increase or decrease.
[0061] Specific examples of the movement path include a spiral
movement path such as that shown in FIG. 3 in which the radius
changes with time and a movement path such as that shown in FIG. 4
including a plurality of concentric circles with different
radii.
[0062] The acoustic matching material 18 with which a container of
the support member 22 is filled is subjected to inertial force due
to the movement of the support member 22. When the support member
22 undergoes linear movement, if the direction is repeatedly
changed, the acoustic matching material 18 may become foamy as a
result of a change in a liquid level due to the inertial force.
Therefore, a location between the object 15 and the plurality of
transducers 17 may not be filled up with the acoustic matching
material. In contrast, when the support member 22 undergoes
circular movement, the acoustic matching material 18 is subjected
to a force in an outer peripheral direction of the circular
movement at all times. Therefore, compared to a movement pattern
formed by the linear movement in which the direction is repeatedly
changed, the circular movement makes it possible to gradually
change the liquid level. Therefore, acoustic matching between the
object 15 and the plurality of transducers 17 is facilitated.
[0063] When the support member 22 is caused to undergo circular
movement, the number of sudden accelerations and decelerations is
small. Therefore, it is possible to restrict the movement of the
acoustic matching material, and to maintain good acoustic matching
between the object 15 and the plurality of transducers 17.
[0064] It is desirable that the scanner 21 move the support member
22 such that the speed in a direction tangent to the movement path
is constant. When the light source 11 is a pulsed light source that
emits light at a constant period, the timing of measuring the
photoacoustic waves 16 is determined by the repetition frequency of
the pulsed light 12 emitted from the light source 11. For example,
if the light source 11 has a repetition frequency of 10 Hz, the
photoacoustic waves 16 can be generated once every 0.1 seconds.
Therefore, if the tangential speed is constant and the
photoacoustic waves 16 are measured every 0.1 seconds, the
measurement positions are spatially uniformly distributed.
[0065] It is desirable that the scanner 21 move the support member
22 from the outer side of the movement plane, in consideration of
the acceleration toward the origin. That is, if the acceleration in
the initial stage of the movement is large, the magnitude of
vibration of the entire apparatus may increase and the vibration
may affect the measurement. Therefore, when the support member 22
starts moving from an outer periphery where the acceleration toward
the origin is small, and then moves towards an inner periphery, the
vibration of the apparatus can be reduced.
[0066] It is desirable that the scanner 21 continuously move the
support member 22, instead of moving it in a step-and-repeat manner
where the support member 22 is moved and stopped repeatedly. This
can reduce the overall time required for the movement and reduce
the burden on a person being examined. Since the change in
acceleration of movement is small, the influence of vibration of
the apparatus or the influence of vibration of the acoustic
matching material 18 can be reduced.
[0067] To move the position of irradiation performed using the
pulsed light 12 generated from the light source 11, it is desirable
that the scanner 21 move the optical system 13 together with the
support member 22. That is, it is desirable that the scanner 21
move the support member 22 and the optical system 13 in synchronism
with each other. Thus, since the relationship between the
photoacoustic-wave measurement position and the light irradiation
position can be kept constant, uniform object information can be
acquired. If the object 15 is a human body, the irradiation area
for irradiating the object 15 is limited by an American National
Standards Institute (ANSI) standard. Therefore, although it is
desirable that the irradiation intensity and the irradiation area
be increased to increase the amount of light propagating to the
interior of the object 15, the irradiation area is limited, from
the viewpoint of, for example, reducing the cost of the light
source. Because of the directionality of transducers, the
efficiency with which light is used is low even if the light is
applied to a region of low reception sensitivity. That is,
irradiating the entire object of large size is not efficient. Since
the efficiency with which light is used is good if light is applied
at all times to a region where the sensitivity of the plurality of
transducers 17 is high, it is desirable to move the scanner 21
while maintaining the positional relationship between the plurality
of transducers and the optical system 13.
[0068] In the embodiment, a light-outgoing portion of the optical
system 13 is disposed at the center (polar portion) of the support
member 22 to apply the pulsed light 12 towards the center of
curvature of the support member 22. This causes light to be applied
at all times to a region where the sensitivity of the plurality of
transducers 17 is high. Since the support member 22 and the optical
system 13 are integrated to each other, it is possible to move a
high-resolution region while maintaining the aforementioned
relationship between the photoacoustic-wave measurement position
and the light irradiation position.
[0069] The computer 19 can control the magnitude of movement, such
as the maximum value of the coordinates r in the radial direction,
the speed of movement (i.e., the radial speed and the tangential
speed), and the way of changing the coordinates in the radial
direction. It is desirable that the maximum value of the
coordinates r in the radial direction be changed in accordance with
the size of the object. For example, when the object is small in
size, the movement of the support member 22 can be controlled with
small coordinates r, whereas when the object 15 is large in size,
the movement of the support member 22 can be controlled with large
coordinates r. This can reduce excess measurement time.
[0070] It is desirable that the photoacoustic apparatus include a
size acquiring unit capable of acquiring information regarding the
size of the object 15. For example, a charge-coupled device (CCD)
sensor capable of acquiring information regarding the shape of the
object 15 may be used as the size acquiring unit. The computer 19
may determine the maximum value of the coordinates r in the radial
direction in accordance with information regarding the size of the
object 15 acquired by the size acquiring unit.
[0071] It is desirable that the photoacoustic apparatus include an
input unit that allows a user to specify a movement parameter, such
as the maximum value of the coordinates r in the radial direction,
for the computer 19.
[0072] By applying the pulsed light 12 at a plurality of timings
after the scanner 21 has moved the support member 12,
high-resolution regions exist at different positions on the basis
of respective measurement timings. As a result, an area of high
resolution regions is expanded. In order to reduce variations in
the resolution in a region that is subjected to imaging, it is
desirable to move the support member 22 so that the plurality of
high-resolution regions overlap.
Distribution of Measurement Positions
[0073] Referring to FIG. 5, a center position (measurement
position) of the support member 22 at each light irradiation timing
(measurement timing) in the photoacoustic apparatus according to
the embodiment is plotted. A point where a vertical line intersects
the support member 22 is defined as the center position, that is,
the measurement position of the support member 22, with the
vertical line extending downward from the center of a
high-resolution region 23 to a movement plane. When the support
member 22 is hemispherical as in the embodiment, the polar portion
of the hemisphere is defined as the center position of the support
member 22. Note that when the position of each transducer at the
corresponding light irradiation timing is plotted, its distribution
is identical to that illustrated in FIG. 5. That is, the
distribution of center positions of the support member 22 can
explain the positions that can be taken by each of the plurality of
transducers 17 at respective measurement timings.
[0074] In the photoacoustic apparatus according to the embodiment,
the computer 19 sets a movement path of the support member 22 and a
light irradiation timing (measurement timing) such that the
measurement positions are distributed as shown in FIG. 5. That is,
the computer 19 sets the movement path of the support member 22 and
the light irradiation timing so that the measurement positions are
distributed in such a manner that the density of the measurement
positions in the movement region becomes constant as shown in FIG.
5. In accordance with the set distribution of measurement
positions, the scanner 21 moves the support member 22, and the
light source 11 generates the pulsed light 12 when the support
member 22 is positioned at a set measurement position. The
photoacoustic apparatus in the present embodiment increases the
density of projections while simultaneously increasing the field of
view (FOV). By adjusting the number of projection angles in
proportion to the projected area encompassed, we were able to
maintain contrast sensitivity and spatial resolution for
arbitrarily large fields of view.
[0075] The photoacoustic apparatus according to the present
embodiment is capable of efficiently and precisely acquiring object
information for each reconstruction position in a region subjected
to imaging.
[0076] For example, with the number of elements and the number of
measurements being the same, as compared to the case of moving a
plurality of transducers whose directional axes are parallel to
each other, the photoacoustic apparatus according to the present
embodiment is capable of acquiring more wave-number information
regarding a photoacoustic wave generated at each reconstruction
position.
[0077] However, due to an error in light irradiation timing, the
arrangement of the plurality of transducers 17, the movement of the
support member 22, or the like, it is difficult to receive
photoacoustic waves having exactly the same wave-number vector
generated from each reconstruction position. Therefore, as long as
photoacoustic waves propagating substantially in the same direction
from each reconstruction position can be received, it is possible
to determine that data having the same relationship in k-space for
each reconstruction position can be obtained. For example, when a
direction is expressed in terms of its solid angle, acoustic waves
that propagate in directions within .+-.4/50.pi.C steradians of the
propagation direction of a photoacoustic wave having a given
wave-number vector can be defined as photoacoustic waves having the
same wave-number vector.
[0078] In the present embodiment, the plurality of transducers 17
are three-dimensionally arranged on the support member 22.
Therefore, even when the support member 22 is moved within a
limited range in two dimensions, acoustic waves propagating in all
directions from each reconstruction position can be received. That
is, by moving the support member 22 as shown in FIG. 5, the
distribution of the positions that can be taken by the plurality of
transducers 17 at all measurement positions becomes a distribution
of positions where acoustic waves (having the same wave-number
vector) that propagate in substantially the same direction from the
respective reconstruction positions can be detected. Therefore,
object information for each reconstruction position can be acquired
by using data of acoustic waves obtained by receiving acoustic
waves having the same relationship in k-space from data acquired at
different measurement timings. This makes it possible to acquire
object information for each voxel using received signal data of
photoacoustic waves that have propagated in all directions from
each voxel. Therefore, the reproducibility of acquired object
information is high. As regards the acquired object information,
since the uniformity of pieces of data used in acquiring the object
information for each reconstruction position of a region that is
subjected to imaging is high, variations in resolution in the
region that is subjected to imaging are small.
[0079] The distribution of measurement positions shown in FIG. 5 is
hereunder described in detail.
[0080] In the present embodiment, the computer 19 sets measurement
positions so as to be distributed spatially uniformly over
360.degree. with respect to the center 24 of the movement plane. In
the present embodiment, the movement plane center 24 refers to the
center of gravity of a region 25 defined by connecting the
measurement positions at an outermost periphery illustrated in FIG.
5.
[0081] The positions that can be taken by a given element in the
plurality of transducers 17 at respective measurement timings are
distributed in a plane parallel to the movement plane of the
support member 22. The positions that can be taken by the given
transducer 17 are spatially uniformly distributed over 360.degree.
with respect to the center of distribution of these positions.
Here, the center of distribution of the positions that can be taken
by the given element refers to the center of gravity of a region
defined by connecting the outermost peripheral positions in the
distribution of the positions that can be taken by the given
element.
[0082] The positions that can be taken by each element of the
plurality of transducers 17 are also distributed in a plane that is
parallel to and different from a plane in which a cluster of
positions that can be taken by the given element selected in the
previous description are distributed.
[0083] That is, spatially uniformly distributing the plurality of
measurement positions over 360.degree. with respect to the center
24 of the movement plane is equivalent to spatially uniformly
arranging each element over 360.degree. in a plane that is parallel
to the movement plane.
[0084] The directions from the respective reconstruction positions
towards the transducers differ from each other. That is, the
transducers receive with good sensitivity different wave-number
vectors of acoustic waves generated at the respective
reconstruction positions.
[0085] That is, the distributions of measurement positions within
four regions (regions 1 to 4) are uniform. The four regions are
defined by two orthogonal straight lines passing through the
movement plane center 24 shown in FIG. 5. In any of the four
regions, the distances between adjacent measurement positions are
equal to each other.
[0086] When the distributions of measurement positions of in the
four regions are compared, the numbers of measurement positions are
the same. In addition, the average distances between adjacent
measurement positions in all the regions are the same.
[0087] Due to error in movement of the support member 22, error in
light irradiation timing, or the like, it is difficult to make the
distances between adjacent measurement positions exactly the same.
The cases where approximately the same effects can be achieved are
also within the scope of the present invention. That is, the case
where the distances between adjacent measurement positions are the
same also includes the case where the distances between adjacent
measurement positions are within .+-.20% of the average distance
between adjacent measurement positions.
[0088] Errors may also occur in the numbers of measurement
positions between regions due to, for example, acceleration or
deceleration of the speed of movement of the support member 22 near
start and end points of the movement of the support member 22.
Therefore, the phrase "the numbers of measurement positions are the
same" means that the average difference in the numbers of
measurement positions between the regions is within .+-.20%. In
addition, the phrase "the distances between adjacent measurement
positions are the same" means that the average difference in
distances of adjacent measurement positions between the regions is
within .+-.20%. That is, if the average difference in the numbers
of measurement positions between the regions and the average
difference in the distances of adjacent measurement positions
between the regions are both within .+-.20%, the distribution of
center positions of the support member 22 in these regions can be
considered uniform.
[0089] It is desirable that the relationship described above be
established even in the four regions defined by any two orthogonal
straight lines passing through the center of the movement plane.
The present embodiment has described the case where measurement
positions are distributed in a plane. The present invention also
includes the case where the movement path extends in a
three-dimensional movement region, and the relationship described
above is established in the four regions defined by two orthogonal
planes passing through the center of the movement region.
[0090] It is desirable that a plurality of measurement positions be
distributed at different positions in any of the regions. This
means that high-resolution regions 23 exist at a plurality of
different positions in the regions. Therefore, variations in
resolution in the regions 1 to 4 are reduced. Thus, it is possible
to suppress an imbalance in resolution depending upon position even
in an entire region that is subjected to imaging.
[0091] It is desirable that a plurality of measurement positions be
set such that each measurement position be spatially equidistance
from at least three measurement positions adjacent thereto so as to
be disposed at equal intervals. By this, in a region that is
subjected to imaging, in an area where there are high-resolution
regions at measurement positions that are distributed at equal
intervals, intervals between adjacent high-resolution regions are
equal, so that variations in resolution are reduced.
[0092] It is desirable that the light source 11 emit the pulsed
light 12 at a constant repetition frequency, and the support member
22 be moved at a constant speed. In this case, pairs of adjacent
measurement positions in the direction of movement along the
movement path are disposed at equal intervals. Additionally, it is
desirable that the movement path and the measurement timing be set
such that the interval between two adjacent measurement positions
along the movement path and the interval between adjacent
measurement positions in a direction that is different from the
movement direction along the movement path be equal.
[0093] However, due to an error in movement of the support member
22, an error in light irradiation timing, or the like, it is
difficult to make the distances between measurement positions
exactly the same. The cases where approximately the same effects
can be achieved are also within the scope of the present invention.
That is, the term "equal interval" refers to the case where
distances to at least three center positions adjacent to a given
measurement region are all within .+-.10% of the average distance
to the at least three measurement positions.
[0094] In the present embodiment, the case in which measurement
positions are distributed in a plane is described. The present
invention also includes the case where the movement path extends in
a three-dimensional movement region, and a measurement position
that is equidistant from at least three measurement positions
adjacent thereto is distributed in the three-dimensional movement
region.
[0095] It is not necessary for the same wave-number component to be
efficiently obtained in all regions that are subjected to imaging.
All that is required is for the same wave-number component to be
efficiently obtained at each reconstruction position of some of the
regions that are subjected to imaging. That is, all that is
required is that there exist a region in which the density of the
distribution of the measurement positions in the movement region is
constant. For example, it is possible for a user to set a region of
interest using the input unit and to set a measurement position
that allows the same wave-number component to be efficiently
obtained for each reconstruction position within the region of
interest. That is, all that is required is to set the measurement
positions so that the density of distribution of measurement
positions for a movement region corresponding to a region of
interest within the movement region is constant.
EXAMPLE
[0096] A photoacoustic apparatus to which the present embodiment is
applied, the photoacoustic apparatus being an apparatus that
realizes photoacoustic imaging, will now be described. A human
breast will be measured in this example.
[0097] The PAI scanner, pictured in FIG. 6, consists of an exam
table (T) upon which a patient lies prone, placing one breast in a
spherically shaped cup (C), thermoformed from a 0.020'' thick sheet
of Polyethylene terephthalate (PETG). A small amount of water is
placed in the breast cup along with the breast prior to imaging to
provide acoustic coupling between the breast and the breast
cup.
[0098] FIG. 7 shows the hemispherical detector array (A), which
lies beneath the cup, affixed to a two-axis translational stage
(XY), whose position is controlled by a pair of
computer-controlled, synchronous motors. The detector array is
comprised of a hemispherical shell (radius=127 mm), machined from
ABS plastic, in which are embedded 512 discrete transducer
elements. Each transducer has a flat active area with a 3-mm
diameter. The center frequency of the 1-3 piezo-composite
transducers is 2 MHz with a 70% bandwidth.
[0099] The detector array and a plastic extension (E) to the array
were filled with degassed, RO water to provide acoustic coupling
between the breast cup and the 512 transducers. A 7-mm-diameter,
pulsed Alexandrite laser beam (75 ns @ 300 mJ/pulse) was fed
through an articulating arm that directed the laser beam (L) upward
along the vertical axis of the transducer array as the array was
scanned. A -12 mm diverging lens, placed at the base of the array,
spread the light in a conical fashion to a diameter of .about.60 mm
at the surface of the breast cup. The peak light fluence was
measured as .about.10 mJ/cm2 at the center of the beam, which is
less than half the maximum permissible exposure (MPE) recommended
by the ANSI.
[0100] A cutaway of the PAI scanner, which shows the geometric
relationships among the detector array, array extension, imaging
table, and breast cup, is illustrated in FIG. 8. The array
extension allows the detector array to be scanned laterally across
the breast surface and still maintain water coupling to the breast.
The maximum imaging volume (1335 mL) is defined by the radius of
curvature of the breast cup (184 mm), the width of the aperture
through which the breast is placed (240 cm) and the maximum
penetration depth for which hemoglobin can be visualized. This
maximum imaging volume is denoted by the hatching in FIG. 8.
[0101] For this PAI scanner we scanned the detector array
continuously in a spiral pattern within a plane that lay normal to
the rotational axis of the array as the laser beam was pulsed at 10
Hz. The spiral patterns we chose were such that the PA data are
acquired at locations within a plane that are spaced equidistantly,
the spacing being the same no matter the size of the spiral. Thus
larger spirals required more pulses and longer image-acquisition
times than smaller spirals. Examples of two of the spiral patterns
used for this work are illustrated in FIG. 9. The smallest spiral
had a maximum radius of 24 mm and consisted of 120 discrete
locations; the larger spiral had a radius of 48 mm and consisted of
480 discrete locations.
[0102] Data from each of the 512 transducers were digitized to 12
bits at 20 MHz for a total of 2048 samples following each laser
pulse. Total data acquisition time was anywhere from 12 seconds for
the smallest spiral to 3.2 minutes for the largest spiral (96 mm).
The lateral field of view (FOV) varied from 100 mm to 240 mm
diameter, depending on the exact spiral pattern chosen.
[0103] Three-dimensional PAI images were reconstructed using a
filtered-backprojection algorithm that has been described
previously. We first measured the PA response to a "point" absorber
fabricated from a small spot of ink placed on the tip of a clear,
thin polyethylene thread, which was then used to calculate a ramp
filter function and simultaneously deconvolve the impulse response
of our transducers. After filtering all our data, we backprojected
our temporal data over spherical surfaces, the radii of which were
determined from the measured speed of the sound of the water
coupling and a second, assumed speed of sound within the breast (or
phantom). We assumed that everything above the breast cup was
homogeneous phantom or breast tissue, and everything beneath was
water. The speed of sound of water was calibrated in our laboratory
as a function of temperature, which was recorded during data
acquisition. We chose the sound speed within the breast (phantom)
interactively by visually assessing the "sharpness" of vessels
within the breast as the assumed sound speed above the breast cup
was varied.
[0104] By using a contrast phantom shown in the upper part of FIG.
10, an experiment was performed to determine the FOV expansion
effect achieved by moving the detector array. It consisted of an
array of 1-mm, dots printed out on a disk of clear plastic with an
HP laser printer and affixed to a 1-cm thick disk of
polyvinylchloride-plastisol (PVCp) to provide rigidity. The
absorbance of the ink at 756 nm was measured with a Genesys 10vis
spectrophotometer as 0.129, which is approximately the same
absorbance one expects for a 1-mm thick sample of blood containing
150 g/L of oxyhemoglobin. We therefore thought it a good surrogate
for blood absorption by a 1-mm-thick blood vessel. We filled the
breast cup with an 8% solution of stock Liposyn-20% to simulate the
attenuation of breast tissue. A photoacoustic image acquired
without moving the detector array is shown in the middle part of
FIG. 10. As can be seen from the photoacoustic image in the middle
part of FIG. 10, the phantom was not entirely imaged and the imaged
region (FOV) was small. Next, the detector array was moved in a
spiral pattern shown in the left part of FIG. 9, and a
photoacoustic image shown in the lower part of FIG. 10 was
acquired. As can be seen from the photoacoustic image in the lower
part of FIG. 10, the image of the entire phantom was acquired and
the FOV was expanded by moving the detector array.
[0105] We quantified the contrast-to-noise performance of the PAI
system by imaging the phantom shown in the upper part of FIG. 10.
We also filled the breast cup with an 8% solution of stock
Liposyn-20% to simulate the attenuation of breast tissue. The
contrast phantom was placed at varying depths within the Liposyn
solution as shown in FIG. 11. PAI images of the phantom were made
using a 24-mm spiral scan (12 seconds). PAI images at three depths
of Liposyn solution are shown in FIG. 12. The contrast and noise
were measured from such images as a function of depth of Liposyn
solution between the breast cup and the location of the contrast
phantom. The contrast and contrast-to-noise ratios were then
plotted as a function of depth of Liposyn solution and analyzed.
The photoacoustic contrast for the center "dot" of our contrast
target (FIG. 11) was measured and plotted as a function of the
depth of the 8% Liposyn solution. The result is plotted in FIG. 13A
and displays a nearly perfect exponential decay with depth. The
noise was calculated as the standard deviation within a small
region of the background in the contrast target. This "noise"
estimate was recorded and used to calculate the contrast-to-noise
ratio (CNR), which is plotted in FIG. 13B.
[0106] We note that the CNR remained constant through the first 3.0
cm of turbid media before dropping, as the depth increased. The
reason for this behavior is due to the dominance of "streak noise",
which is much greater than the electronic noise "floor" of the
input electronics of our data acquisition system at shallow depths.
As the PAI contrast falls to lower levels at greater depths, this
electronic noise begins to degrade our CNR as can be seen in FIG.
13. We measured the spatial resolution of the PAI scanner by
imaging a long filament of graphite fiber embedded in an agar mold,
shaped to fit snugly into our breast cup. A MIP calculated from a
3D PAI image (48-mm spiral scan, 48 seconds) are shown in FIG. 14.
The full-width half-maximum (FWHM) of a profile across one of the
carbon fibers was used to estimate the spatial resolution of the
PAI system. The spatial resolution was estimated from a plot across
one filament of the graphite fiber phantom (FIG. 14). The
full-width, half-maximum of the plot was 0.42 mm.
[0107] The resolution was kept almost constant throughout
substantially the entire region within the FOV.
In order to demonstrate that we can maintain good contrast
sensitivity at the periphery of the field of view, we translated
our contrast phantom laterally 80 mm and scanned with our largest
spiral protocol (96 mm, 3.2 minutes). The lateral field of view was
measured as 24 cm as indicated from the visibility of the contrast
phantom placed at the edge of our field of view (FIG. 15). The
lateral field of view was measured as 24 cm as indicated from the
visibility of the contrast phantom placed at the edge of our field
of view (FIG. 15).
[0108] We chose to scan the hemispherical detector array
continuously in a spiral pattern during data acquisition, rather
than rotating the array about its vertical axis as had been done in
our previous prototype PAT scanner. This new spiral scan protocol
increased the density of projections while simultaneously
increasing the FOV. By adjusting the number of projection angles in
proportion to the projected area encompassed by the spiral scan, we
were able to maintain contrast sensitivity and spatial resolution
for arbitrarily large fields of view as needed in breast
imaging.
Other Embodiments
[0109] Embodiments of the present invention can also be realized by
a computer of a system or apparatus that reads out and executes
computer executable instructions recorded on a storage medium
(e.g., non-transitory computer-readable storage medium) to perform
the functions of one or more of the above-described embodiment(s)
of the present invention, and by a method performed by the computer
of the system or apparatus by, for example, reading out and
executing the computer executable instructions from the storage
medium to perform the functions of one or more of the
above-described embodiment(s). The computer may comprise one or
more of a central processing unit (CPU), micro processing unit
(MPU), or other circuitry, and may include a network of separate
computers or separate computer processors. The computer executable
instructions may be provided to the computer, for example, from a
network or the storage medium. The storage medium may include, for
example, one or more of a hard disk, a random-access memory (RAM),
a read only memory (ROM), a storage of distributed computing
systems, an optical disk (such as a compact disc (CD), digital
versatile disc (DVD), or Blu-ray Disc (BD).TM.), a flash memory
device, a memory card, and the like.
[0110] While the present invention has been described with
reference to exemplary embodiments, it is to be understood that the
invention is not limited to the disclosed exemplary embodiments.
The scope of the following claims is to be accorded the broadest
interpretation so as to encompass all such modifications and
equivalent structures and functions.
[0111] This application claims the benefit of U.S. Patent
Application No. 61/873,542, filed Sep. 4, 2013, which is hereby
incorporated by reference herein in its entirety.
* * * * *